This application claims the priority of German Patent Application No. 197 31 642.5, filed Jul. 23, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a fuel cell vehicle, for example, an automobile.
Fuel cell vehicles having an electric drive system and a fuel cell system for providing electric energy for the drive system are generally known. The electric drive system typically has one or more electric motors that convert the electric energy generated by the fuel cell system into driving forces for the vehicle wheels. Frequently, the fuel cell system of known fuel cell vehicles comprises a methanol reforming system by means of which hydrogen for feeding the fuel cells is obtained from methanol carried along in a liquid state so that a large-volume hydrogen storage device is not necessary.
A fuel cell system of this type is described in German Published Patent Application DE 33 45 956 A1. In the system described therein, water electrolysis devices are also provided by means of which, during vehicle operating phases with a low or normal load, a portion of the energy generated by the fuel cell system is used for the electrolysis of water, in which case the hydrogen and oxygen obtained in this manner are stored in a respective pressure accumulator. During vehicle operating phases with a peak load demand, the stored hydrogen and oxygen are then fed to the fuel cells as reaction gases instead of a methanol reformate gas and air, which in the other operating phases are used as reaction gases. As a result, the size of the fuel cell stack can be designed for lower power requirements during the normal operation and can nevertheless meet peak performance demands.
In the case of vehicles having a hybrid drive, it is known to set up the electric drive system for the regenerating of braking energy, for example, an electric machine of the drive system is alternatively used as an electric motor and as a generator. In operating phases in which forces are to be transmitted to the driving wheels, the electric machine operates as a driving electric motor while it operates as a generator in braking phases. Thus, kinetic vehicle energy is reconverted into electric energy. The braking energy regenerated in this manner is intermediately stored in an energy storage device, such as a flywheel storage device or a buffer battery, and is therefore available again as driving energy. A hybrid drive of this type is described in German Published Patent Application DE 41 24 479 A1. It is also known to feed the regenerated braking energy of a hybrid drive system alternatively not into an accumulator but to use it directly for the feeding of electric consuming devices, such as lights, fans or an air conditioner compressor. See German Published Patent Application DE 41 42 863 A1.
A fuel cell vehicle of the initially mentioned type is described in German Published Patent Application WO 96/41393 A1. The fuel cell system described there contains, among other things, a tempering fluid circulation system for regulating the temperature of the fuel cell stack. An electric resistance is situated in the tempering fluid circulating system by means of which the tempering fluid can be heated. The resistance is electrically fed by way of a power supply bus into which the electric energy generated by the fuel cell stack is fed and to which an electric vehicle driving motor is coupled by way of an inverse rectifier. By way of the electric driving motor, which will then operate as a generator, and the inverse rectifier, vehicle braking energy can be fed back into the power supply bus and can be utilized for energizing the resistance and thus for heating the tempering fluid when and if there is a demand for such a tempering fluid heating during a braking phase. In order to permit a heating of the tempering fluid, for example when the vehicle is stationary, it is also possible to connect the power supply bus to an external power supply system. Hydrogen which is obtained, for example, from a reformation reaction can be used as a fuel for the fuel cell stack. For providing the hydrogen, a methanol reforming system can be used, for example, and the oxygen supply of the fuel cells can take place in the form of an oxygen-containing gas that is compressed by means of a compressor. See the journal essay by K. B. Prater, xe2x80x9cSolid Polymer Fuel Cell Developments at Ballardxe2x80x9d, Journal of Power Sources, 37 (1992), Page 181.
The present invention is based on the technical problem of providing a fuel cell vehicle that has a comparatively high degree of energy utilization.
In the case of the present fuel cell vehicle, the electric driving system is set up for regenerating the braking energy, and devices are provided by means of which a direct utilization of the regenerated braking energy is implemented in at least one energy-consuming component of the fuel cell system. In this manner, the kinetic vehicle energy during braking operations is not completely lost but can be utilized at least partly in an energy-consuming component of the fuel cell system, which increases the degree of total energy utilization of the vehicle. Since the regenerated braking energy is utilized directly for the fuel cell system, no additional intermediate energy storage device, for example a buffer battery or traction battery, is required, which contributes to the saving of energy and permits a braking energy regenerating without any problem. In addition, it is found that this type of utilization of the regenerated braking energy is clearly simpler and presents fewer problems than a theoretically possible utilization in the form of a reversal of the fuel cell reaction in order to regenerate by means of this reversal reaction hydrogen and oxygen by electrolysis. Because of the materials and catalysts normally used in the fuel cells, this reversal reaction presents problems in practice.
In the case of a vehicle according to a first embodiment of the present invention, regenerated braking energy is utilized specifically for a compressor by means of which oxygen-containing gas, such as air, is compressed for the fuel cells. In this case, the compressor may contain an integrated flywheel storage device that receives the fed braking energy, or a pressure accumulator is provided in which, when regenerated braking energy is fed to the compressor, additional compressed gas is stored intermediately.
In the case of the vehicle according to a second embodiment of the present invention, regenerated braking energy is utilized specifically for the intermediate storage of a hydrogen-containing reformate gas that is supplied by a reforming reactor unit of a methanol reforming system. For this purpose, a compressor provided downstream of the reactor unit is operated by means of regenerated braking energy. The compressor thereby compresses reformate gas which is then intermediately stored in a pressure accumulator provided for this purpose and is available for operating phases with an increased power demand.
In the case of the vehicle according to a third embodiment of the present invention, the regenerated braking energy specifically in a methanol reforming system as part of the fuel cell system, is utilized in that, by means of an evaporator, a water/methanol mixture is evaporated at an excess and the excess water/methanol vapor is intermediately stored in a vapor accumulator which is provided for this purpose and which is arranged between the evaporator and a reforming reactor unit connected behind it.
In the case of a vehicle according to a fourth embodiment of the present invention, regenerated braking energy is fed to a reactor heater and is utilized for additional heating of a reforming reactor unit by means of which methanol is reformed in order to obtain hydrogen for the fuel cells. This additional heating of the reforming reactor unit promotes the conversion of methanol, in which case, as the result of the heat capacity of the reactor unit, an intermediate energy storage effect is achieved.
In the case of a vehicle according to a fifth embodiment of the present invention, regenerated braking energy is utilized for the additional heating of an evaporator of a methanol reforming system by means of which a methanol/water mixture is evaporated before the feeding into a reforming reactor unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.